Quick answer: A 25kW combi suits most South London properties — a flat or Victorian terraced house with one bathroom and up to 10 radiators. A 30kW combi is the right call when you have two bathrooms or 10–15 radiators and expect simultaneous hot water demand. The deciding factor is never heating capacity. It is always hot water flow rate.
Why the kW rating is almost entirely about hot water, not heating
Most homeowners assume a higher kW boiler heats a home faster or more powerfully. That is not how combi boilers work.
For central heating alone, a well-insulated modern property rarely draws more than 8–12kW continuously. Even in a cold snap, the radiator circuit is not the load that sizes the boiler.
The load that sizes the boiler is domestic hot water. A combi heats water on demand — no cylinder, no stored reserve. The kilowatt rating determines how fast cold mains water can be heated to usable temperature. That speed is measured as flow rate in litres per minute (L/min).
- 25kW → ~10 L/min at a 35°C temperature rise — one shower running comfortably and stably
- 30kW → ~12 L/min at a 35°C temperature rise — shower plus basin simultaneously, or a bath filling faster
A comfortable power shower needs 8–10 L/min. A 25kW combi delivers that easily for one outlet at a time. The 30kW advantage only becomes relevant when two people want hot water from different outlets at the same moment — one in the shower upstairs, one washing up or running a bath.
What size combi boiler do I need?
Match your property type to the correct output:
- 1–2 bed flat or terraced house (up to 10 radiators, 1 bathroom) → 25kW
- 3 bed terraced or small semi (10–12 radiators, 1 bathroom) → 25kW
- 3 bed with en-suite shower room as well (both rarely used simultaneously) → 25kW or 30kW — see note below
- 3–4 bed semi or detached (12–15 radiators, 2 bathrooms) → 30kW
- Large detached or multi-bathroom property (15+ radiators, 3+ bathrooms) → system boiler with cylinder — combi is not appropriate
The borderline case: A 3-bed terraced with one main bathroom and a small en-suite can go either way. If the en-suite and main bathroom are never in use simultaneously — true for most households with children — 25kW is correct. If adults in the house regularly shower at the same time, step up to 30kW.
The typical stock Reion works on across Wandsworth, Lewisham, Lambeth, Southwark, and Bromley is Victorian and Edwardian terraced housing: 2–3 beds, one main bathroom, 8–12 radiators. The right answer for that property type is a 25kW, nine times out of ten.
Why oversizing causes real problems, not just theoretical ones
Fitting a 30kW boiler into a small terraced house does not give you extra performance. It gives you short cycling.
Short cycling happens when a boiler produces more heat than the radiator circuit can absorb. The boiler fires, the water temperature reaches its upper limit quickly, the burner shuts off, the system cools, the burner fires again — in 3–5 minute bursts rather than 15–20 minute runs.
The consequences:
- Efficiency drop of 5–10% compared to a correctly sized unit running steady burn cycles
- Accelerated component wear — gas valve, heat exchanger, and ignition components are the parts most affected and most likely to generate fault codes
- Higher gas bills — visible on a smart meter within the first winter
If your boiler is generating repeated fault codes on a system that otherwise looks clean, short cycling from an oversized original installation is sometimes the root cause. A boiler service can confirm whether cycling is occurring and how severe it is.
The one thing a bigger boiler cannot fix: mains pressure
If your shower pressure is weak, a 30kW boiler will not solve it.
Flow rate from a combi and mains water pressure are separate variables. The boiler determines how fast it heats the water. The street mains determines how much water actually arrives at the boiler.
Thames Water's statutory minimum supply pressure is 1 bar at the point of entry. Across South London, typical static mains pressure runs 1.5–3 bar — but it is not uniform. Higher-elevation postcodes (parts of Wimbledon, Crystal Palace, Sydenham Hill, Tulse Hill) tend to sit at the lower end. River valley areas (Wandsworth town centre, Vauxhall, Deptford) generally have stronger supply.
If your postcode is on elevated ground and mains pressure is already marginal, a 30kW combi running at 12 L/min will simply be throttled by the supply. You will spend more and feel no difference at the showerhead.
Reion checks incoming mains pressure as part of every new boiler installation consultation — it takes two minutes with a pressure gauge and prevents exactly this mistake.
Which boilers we recommend at each output
These are the models Reion installs most frequently across South London. Full fault code guides and reliability notes are in the boiler section of this site.
25kW recommendations:
- Worcester Bosch Greenstar 25i — most reliable parts chain in the UK, 12-year warranty available
- Vaillant ecoTEC Plus 25 — strong efficiency rating, widely serviced across South London
30kW recommendations:
- Worcester Bosch Greenstar 30i or CDi — same reliability platform at higher output
- Viessmann Vitodens 100-W 30kW — premium build quality, lower long-term fault rate
Avoid upsizing to a 30kW model from a brand with harder parts sourcing — Alpha and Ferroli in particular. If those brands need a gas valve or heat exchanger at an oversized output, you compound parts lead time with an already higher fault rate.
Get a quote for a correctly sized boiler installation
Reion covers Wimbledon, Merton, Wandsworth, Lewisham, Lambeth, Southwark, Greenwich, Bromley, and North Surrey. Get a quote here — the form takes two minutes and Reion responds personally. Every installation quote confirms the correct boiler size for your property before any work begins.
Gas Safe registered engineer · Registration 919881 · All work carries a 12-month workmanship guarantee
